The referenced U.S. Pat. No. 3,949,401, Hegeler et al, describes a radio transmission system, and a receiver for use therein, which is capable of decoding transmissions radiated on a separate subcarrier to indicate that a radio station is about to make, or making, a special announcement, for example a traffic warning, accident or other emergency reports, or, if desired, sports or news announcements, or the like. The receiver, of course being tuned to the particular station, may not have previously reproduced the radiated program, however, but, rather, may have only used its audio section for reproduction, for example, of an audio program previously recorded on magnetic tape and being reproduced from a magnetic cassette or cartridge which is scanned by an appropriate transducer section of a combination cassette/cartridge radio receiver. Thus, the user, who may not be interested in the particular program being broadcast by the transmitter, can listen to the program in accordance with the user's preference, as determined by reproduction from the tape--or, for example, from a different radio station with a separate tuner--although the user may be interested in receiving all the announcements being broadcast by the radio station. Under ordinary operation of the system, that is, if no special announcement is being radiated, the audio portion of the receiver is muted. The audio amplification stage may be used for reproduction of a tape-recorded program, or may be muted as well.
It has previously been proposed to monitor the capability of the muted radio receiver to receive radiated signals; German Pat. No. 24 28 131 describes such a system. In the system of this German patent, a manually operated switch is provided which, in case the radio receiver is muted, places a voltage on an indicator panel which includes indicator lamps if an announcement recognition signal is detected by a decoder coupled to the tuner of the radio receiver.
The switch permits, or blocks automatic cancellation of muting. This cancellation of muting is, normally, controlled by the announcement recognition signal, as decoded by the decoder. For details of the decoding, reference is made to the aforementioned Hegeler patent. The manually operated switch permits--upon operation--that the muting circuit is enabled, rather than being automatically disabled by decoding of the announcement recognition signal, and the receiver muted--or, for example, a tape-recorded program again enabled if the user is not interested in listening to the specific announcement. Since, frequently, announcements are repeated from time to time, or are played from an automatically repeating tape, the listener, after once having heard an announcement, may wish to revert to complete muting, or another program. Yet, the indicator lamp will indicate that an announcement continues to be broadcast, in case the listener wishes to hear it again, and, by a second operation of the switch, inhibit the muting of the receiver.
Traffic announcement system in which the receiver can be used have been developed to discriminate between various types of announcements; the respective types of announcements are assigned separate modulating frequencies which, by amplitude modulation (AM) modulate a subcarrier, typically of 57 kHz, which is frequency modulated on the main carrier of the frequency modulation (FM) transmitter station to which the receiver is tuned. One of the modulating frequencies (f.sub.0) may be assigned warning or emergency announcements, which should be heard by all listeners; other frequencies (f.sub.1 ; . . . ) may be assigned different announcements, for example traffic announcements or sports or news reports. For a detailed description, reference is made to the referenced application U.S. Ser. No. 319,653, Eilers et al.
It is, of course, necessary that the receiver be operable to receive warning or emergency information. It may happen that, for an extended period of time, no warning or emergency information will be radiated. It is no possible to test the receptivity of the receiver, then, to such warning information. Of course, it would be possible to repeatedly, for example at recurring intervals, radiate the particular frequency (f.sub.0) associated with warning information. To test receptivity, however, it will be necessary that the user be present, and has the radio connected at the time this test signal is being radiated-which cannot always be presumed.